Programmable controllers are well known devices for operating industrial equipment such as assembly lines and machine tools, in accordance with a stored program. In such controllers, the control program is stored in a memory and includes instructions which are read out in rapid sequence and executed to examine the condition of selected sensing devices on the controlled equipment, and to energize or deenergize selected operating devices on the controlled equipment contingent upon the status of one or more of the examined sensing devices. Other instructions are provided not only to manipulate single-bit input and output data representing the state of the sensing and operating devices, but also to perform arithmetic operations, timing and counting functions, and more complex processing operations.
In carrying out its functions, programmable controller processor is expected to execute certain well-known programmable controller type instructions. Such instructions have become quite standardized in the industry and they are directly associated with the elements of a ladder logic diagram which is easily understood by control engineers. Program panels such as those described in U.S. Pat. Nos. 3,798,612; 3,813,649 and 4,070,702 have been developed to assist the user in developing and editing ladder logic control programs comprised of such programmable controller instructions. Another type of machine controller is a sequencer, such as a mechanical rotating drum sequencer of the type which controls washing machines. This mechanical type device has a program drum, or cylinder, which rotates. A series of projecting tabs on the drum are aligned to make or break electrical switch contacts as the drum rotates in a predefined direction. Thus, as the drum rotates, stepping through a set sequence, various combinations of outputs may be energized or deenergized. The conventional sequencer has been implemented as a programmable electronic device, such as the one described in U.S. Pat. No. 3,974,484. The traditional sequencer has the advantage that in the event of an error condition, the state of the equipment that prompted the error condition are easily determined. One drawback of this type of controller is that the control must always advance from each step to the same subsequent step. In many control processes, however, it is necessary to advance from one step to one of several subsequent steps depending upon the state of several parameters of the control process.
It is therefore advantageous to be able to utilize a sequencer type control device with its ability to construct an error history with a programmable controller. Such a hybrid type device would enable transitions from a given step to one of several subsequent steps depending upon the state of various input conditions being sensed by the programmable controller.